The present invention relates to a method for solidifying high and medium radioactivity and/or actinide containing aqueous waste concentrates or fine-grained solid wastes suspended in water for final noncontaminating storage in which the waste concentrates or the suspensions are subjected, together with absorbing and/or hydraulically binding inorganic materials, to a ceramic firing process so as to produce a solid sintered body.
It has been known for a long time to solidify radioactive aqueous solutions by first reducing the volume of such wastes, thereby concentrating the radioactive substances, and then treating the concentrates either by (1) subjecting them together with glass formers to a heat treatment until the radioactive substances become distributed throughout the resulting glass melt and then having the melt solidify into a solid body, or (2) by mixing the concentrated wastes with silicate-containing clays or with ion exchangers, respectively, and firing the resulting mix ceramically so as to form a solid body.
Some of the drawbacks of producing glass blocks having radioactive waste substances incorporated therein include the need to use relatively complicated and expensive apparatus which must be operated by trained personnel. Moreover, in the course of prolonged storage, decomposition of the glass structure may occur due to the continued emission of radiation and heat energy by the incorporated highly radioactive substances with the result that the resistance of the glass structure to leaching deteriorates with time and its ability to effectively retain radioactive materials is diminished, especially as compared to the relatively good leaching properties of nondecomposed glass waste blocks.
When clay-radionuclide mixtures are fired according to the prior art, the quality of the solidified products containing high concentrations of radioactive substances has not been sufficient for final storage purposes.
An additional problem encountered with prior art solidification by glass and fired clay processes is that during the high temperature stages, significant quantities of radioactive substances evaporate from the not yet solidified waste. These escaping impurities must be trapped and removed by complicated waste gas purification techniques involving solids filters, gas washing columns and condensate separators.
German Pat. No. 1,127,508 to Alberti proposes mixing aqueous atomic waste with fireproof cement and then increasing the density of the resulting hardened block by ceramic firing to produce a sintered body which is resistant to leaching. In order to increase the mechanical stability of the hardened block, the patent suggests adding fireproof additives such as fire clay or brick chips to the fireproof cement. For example, a cylindrical molded body was produced from molten alumina cement and radioactive liquid. The molded body, after hardening, was uniformly heated for a period of 5 hours to a temperature of 500.degree. C. to evaporate excess water. The molded body was then rapidly brought to a firing temperature of, for example, 1100.degree. C. and kept at this temperature for about 2 to 4 hours after which the molded body was cooled slowly. No information is given in the patent about the radioactivity of the radioactive liquid being treated. There is also no disclosure in the patent as to the quantities of liquid being treated in the 3-liter vessels used by Alberti or as to the water-cement values, or the like. Results of leaching experiments likewise were not disclosed.
The Alberti process may be useful for the solidification of low radioactivity aqueous wastes, but it is very expensive and unnecessarily complicated. Further, it cannot be used for the solidification of high or medium radioactivity and/or actinide containing aqueous wastes.
Medium activity waste solutions have been solidified in cement, concrete or bitumen at temperatures of more than about 150.degree. C. Solidification of medium activity waste solutions with cement, concrete or bitumen leads to end products which have low thermal stability and relatively low radiation resistance over extended periods of time. As a result, special safety measures become necessary when depositing these products for intermediate or final storage.
When seeking to store actinide concentrates, the intensive development of radiolysis gases and heat in the product, as a result of the radioactive decomposition of the actinides, renders bitumen, cement or concrete solidification processes completely unsuitable.
Suspended combustion ashes or ion exchangers have previously been solidified in cement and put into barrels which act as sheaths. The thus sheathed, solidified products have then been put directly into storage. The properties of such blocks, however, particularly with respect to mechanical stability and leaching resistance, are not particularly good, so that this type of solidification is used only for weakly active wastes.